Properties of konjac glucomannan–whey protein isolate blend films

Properties of konjac glucomannan–whey protein isolate blend films

LWT - Food Science and Technology xxx (2014) 1e7 Contents lists available at ScienceDirect LWT - Food Science and Technology journal homepage: www.e...

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LWT - Food Science and Technology xxx (2014) 1e7

Contents lists available at ScienceDirect

LWT - Food Science and Technology journal homepage: www.elsevier.com/locate/lwt

Properties of konjac glucomannanewhey protein isolate blend films Manusawee Leuangsukrerk, Thunyaluck Phupoksakul, Kanitha Tananuwong, Chaleeda Borompichaichartkul, Theeranun Janjarasskul* Department of Food Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 December 2013 Received in revised form 8 May 2014 Accepted 12 May 2014 Available online xxx

Edible composite packaging has been developed by blending biocomponents for specific applications, aiming to take advantage of complementary functional properties or to overcome their respective flaws. The aim of this work was to study the effect of incorporation of whey protein isolate (WPI) on the properties of konjac glucomannan (KGM) based films. Five aqueous solutions of KGM and/or WPI were prepared by casting and solvent evaporation of 1:0, 0.8:3.4, 0.6:3.6, 0.4:3.8 and 0:4.2 g KGM:g WPI/100 g solution. Glycerol (Gly) was used as a plasticizer at 1.5 and 1.8 g/100 g solution. The result showed that incorporated WPI proportionally increased transparency of KGM-based films. An increase in proportion of WPI resulted in decreased tensile strength and elastic modulus as well as improved flexibility. The incorporation of WPI into the KGM matrix led to an increase in water insolubility which enhanced product integrity and water resistance. Nevertheless, WPI did not improve water vapor barrier of KGM eWPI films. WPI and blend film with the highest concentration of WPI could be heat sealed at 175  C. Overall, the range of Gly in this study did not apparently affect properties of the films. © 2014 Elsevier Ltd. All rights reserved.

Keywords: Edible film Konjac glucomannan Whey protein isolate Tensile strength Water vapor permeability

1. Introduction Advanced research activities in edible packaging material have been driven by both increasing consumer demands for stable, safe, convenient, high quality foods as well as ecological consciousness. Edible films have been developed as food packaging materials because of their ability to safely protect food products by providing barrier properties, controlled releasing food ingredients, enhancing mechanical integrity of foods while reducing the environmental impact (Janjarasskul & Krochta, 2010). Konjac glucomannan (KGM) is a high molecular weight and water soluble linear polysaccharide extracted from the tubers of Amorphophallus konjac plant. The main chain comprises of b-1, 4 linked D-mannose and D-glucose with a molar ratio of 1.6:1, with approximately 1 acetyl group at C6 position in every 17e19 sugar units (Liu & Xiao, 2004). KGM film is flexible despite the absence of small molecular mass plasticizers, easy to be peeled from the casting plate, and enhanced water vapor barrier compared to other polysaccharide films (Cheng, Abd, Norziah, & Seow, 2002). However, KGM film is readily soluble in water which is an important obstacle for establishing water resistant packaging. Furthermore,

* Corresponding author. Tel.: þ66 81 9209474; fax: þ66 2 881 0618. E-mail address: [email protected] (T. Janjarasskul).

KGM film is translucent and low in gloss, which is typically opposite to preferred visual properties by consumers. Whey proteins are a mixture of globular proteins isolated from whey, a by-product of cheese-making process. There are two commercial forms available; whey protein concentrate (WPC, 25e80 g protein/100 g) and whey protein isolate (WPI, >90 g protein/100 g) (Gennadios, 2002). WPI film is transparent, colorless, tasteless, odorless, heat-sealable, with good oxygen barrier and tensile properties at low to medium relative humidity (RH) conditions (McHugh & Krochta, 1994). Edible composite packaging materials have been developed by blending biocomponents for specific applications, aiming to take advantages of complementary functional properties or to overcome their respective flaws. Many biopolymer films have good mechanical properties or good physical properties but not both. Therefore, the method to overcome their drawbacks is through blending of biopolymers. Blending of KGM-based films with other biopolymers such as starch (Chen, Liu, Chen, Chen, & Chang, 2008), gellan gum (Xu, Li, Kennedy, Xie, & Huang, 2007), and gelatin (Li, Kennedy, Jiang, & Xie, 2006) have been reported. Chen et al. (2008) found that there was a good miscibility between KGM and starch. The elongation of starcheKGM blend films was higher than the KGM film. However, blending WPI into KGM film has never been explored. Furthermore, one of the major drawbacks of edible polysaccharide films from commercial applications is the ability to

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Please cite this article in press as: Leuangsukrerk, M., et al., Properties of konjac glucomannanewhey protein isolate blend films, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.05.029

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be heat sealed. On the contrary, WPI displays thermoplastic behavior at low moisture level (Hernandez & Krochta, 2008). WPI have been successfully formed into films and sheets using thermoplastic processes such as compression molding and extrusion (Sothornvit, Olsen, & Krochta, 2007) as well as reported to be heat sealable (Kim & Ustunol, 2001). Therefore, we hypothesized that the incorporation of WPI could improve properties; including appearance, water vapor barrier and heat sealability, of KGM films. Thus, the objective of this research was to study the effect of incorporating WPI on the transparency, tensile properties, water insolubility, water vapor barrier and heat sealability of KGM based film. Effect of glycerol content on these properties of KGMeWPI blend films was also investigated. 2. Materials and methods 2.1. Materials Whey protein isolate BiPRO® (97.0 g protein/100 g) was supplied by Davisco Foods International, Inc. (Le Sueur, Minn., USA). Konjac glucomannan, molecular weight ranges from 200,000 to 2,000,000, was purchased from Yunnan Gengyun Konjac Resources Developing Co., Ltd. (Kunming, China). Glycerol QReC™ (99.5% purity), used as a plasticizer to improve film flexibility, was purchased from QReC Chemical Co., Ltd. (Chonburi, Thailand).

measurements were made for each film replicate. The transparency of the films was calculated by the following equation:

Transparency value ¼ log T560 =x where T560 is the transmittance at 560 nm and x is the film thickness (mm). 2.5. Mechanical properties An Instron universal testing machine (Model 5565, Instron, Norwood, USA) was used to measure tensile strength (TS), percent elongation (%E), and elastic modulus (EM) according to ASTM D882: the standard test method for tensile properties of thin plastic sheeting (ASTM, 1997a). The test films were cut into 30 mm wide and 120 mm long strips. The initial distance between the grips was 50 mm. The cross-head speed was 5.0 mm/s and the load cell was 5 kg. Tensile strength was calculated by dividing the maximum load by the original cross-sectional area of the test film. The result was expressed in MPa. Elastic modulus was calculated from the slope of the initial linear portion of the stressestrain curve. The result was expressed in MPa. Percent elongation was calculated by dividing the elongation at the moment of rupture of the test film by the initial gauge length and multiplying by 100. The result was expressed in percent. A total of 5 measurements were made for each film replicate.

2.2. Film preparation and formation 2.6. Solubility One g KGM/100 g solution and 4.2 g WPI/100 g solution were prepared by adding KGM or WPI powder slowly into distilled water. Three mixture solutions were prepared by varying KGM at 0.8, 0.6 and 0.4 g and WPI at 3.4, 3.6 and 3.8 g per 100 g solution. The total biopolymer content of KGM and WPI in the mixture solution is 4.2 g biopolymer/100 g solution. Mixture solutions of KGM and WPI were prepared by slowly adding WPI and KGM powder, respectively, under constant stirring using a magnetic stirrer at room temperature for 75 min in order to totally dissolve both WPI and KGM, yielding homogenous mixture. The casting solutions were then denatured in a water bath at 90  C for 30 min and cooled down immediately to the room temperature in an ice bath. Glycerol (Gly) was later added to the solutions as plasticizer at 1.5 or 1.8 g/100 g solution. Casting solutions, containing 6 g of total solids, in order to maintain film thickness as constant as possible, were poured onto acrylic plates with casting dimension of 15 cm  30 cm. Cast films were allowed to completely dry in a tray dryer at 50  C for 15 h. Dried films were peeled off intact from the surface of the plates before conditioning at 50% RH and 25  C for at least 48 h prior to tests.

A 2 cm  2 cm test films were immersed in 25 ml of distilled water for 24 h with slow mechanical stirring at room temperature. The filtrated film residues were dried in a forced air oven (105  C for 24 h) to constant weight. The initial dry weight was determined from the sample moisture content. Percent total soluble matter was calculated from the initial and final dry weight of films and reported on dry weight basis. A total of 3 measurements were done for each film replication. 2.7. Water vapor permeability Water vapor permeability (WVP) was determined according to ASTM E96: the standard test method for water vapor transmission of materials (ASTM, 1989). Films were cut and mounted on test cups containing 6 ml of water. The cups were placed in desiccator cabinets containing fans and held at 0% RH using silica gel. Weight was taken after steady state was achieved. Changes in weight of the cell were plotted as a function of time. The water vapor transmission rate (WVTR) was calculated from the slope of the constant rate of weight loss divided by film area. WVP was calculated from the following equation:

2.3. Thickness Thickness of the films was measured using a digital micrometer (Model ID-C112, Mitutoyo MFG Co. Ltd., Kanagawa, Japan) at 10 random positions of each test film. Mean values were used for the calculations of transparency, mechanical and barrier properties.

WVP ¼

 WVTR g h1 m2 *thickness ðmmÞ p1  p2 ðkPaÞ

where p1 is water vapor partial pressure at the film inner surface (kPa), and p2 is water vapor partial pressure at the film outer surface (kPa).

2.4. Transparency 2.8. Thermal transitions The transmittance of the films was measured by a UVevisible spectrophotometer (Model Genesys 10, Rochester, New York, USA) at 560 nm according to ASTM D1746: the standard test method for transparency of plastic sheeting (ASTM, 1997b). The test films were cut into rectangular shape of 1 cm  4 cm, then attached onto the surface of a cuvette. An empty cuvette was used as a reference. Five

Thermal transitions were determined using a differential scanning calorimeter (DSC) (Diamond, Perkin Elmer, Massachusetts, USA) equipped with an Intracooler 2P (Perkin Elmer, Massachusetts, USA) and nitrogen gas purge. The film was cut to the disc of approximately 0.5 cm diameter. Each disc weighed 10e15 mg. The

Please cite this article in press as: Leuangsukrerk, M., et al., Properties of konjac glucomannanewhey protein isolate blend films, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.05.029

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disc samples were hermetically sealed in a large volume stainless steel pan (60 mL). The samples were heated from 55  C to 200  C at a rate of 10  C/min. An empty stainless steel pan was used as a reference. The onset (To), peak temperature (Tp) and enthalpy (DH) of the endotherm were determined via Pyris™ version 8 software (Perkin Elmer, Massachusetts, USA). 2.9. Seal strength Film samples were cut into strips of 7.62  2.54 cm. Two films strips were placed on top of one another, and an area of 2.54  1.5 cm (at the edge of the film) was heat-sealed by MULTIVAC Vacuum packer (Model C350, MULTIVAC SeppHaggenmüller GmbH & Co. KG, Wolfertschwenden, Germany) at 175  C with heating time of 2.7 s and dwell time of 5 s allowing sealing area to cool down completely. All sealed film samples were conditioned at 50% RH, 48 h prior to determining seal strength. The seal strength was measured using an Instron universal testing machine according to ASTM F88: the standard test method for seal strength of flexible barrier material (ASTM, 2005). Each leg of the sealed film was clamped to the machine, with each end of the sealed film held perpendicularly to the direction of the pull. Seal strength (N/m) was calculated using the following equation

Seal strength ¼

Peak force Film width

2.10. Statistical analysis All property tests were replicated three times. Data were analyzed by completely randomized design (CRD) and were presented as mean values with standard deviations. Differences between mean values were established using Duncan's new multiple range test at a confidence level of 95%. 3. Results and discussion 3.1. Film formation KGM solutions were partially opaque and very high in viscosity, which in turn limited the maximum concentration of KGM in casting solution to 1 g/100 g solution. KGM hydrates rapidly by absorbing up to 200 times their weight in water to form viscous, pseudoplastic dispersions (Nishinari, Kim, & Kohyama, 1987) which lend itself to be commonly used as thickening agent, stabilizer, or gelling agent (Zhang, Xie, & Gan, 2005). On the other hand, WPI quickly dissolved to form thin, transparent, slightly yellowish solution. The maximum WPI concentration was limited to 12 g/100 g

Table 1 Thickness of konjac glucomannan (KGM), whey protein isolate (WPI) and blend films. (g KGM:g WPI)/100 g solution

Thickness (mm) Glycerol (g Gly/100 g solution) 1.5

1:0 0.8:3.4 0.6:3.6 0.4:3.8 0:4.2

0.128a 0.128a 0.121abc 0.126ab 0.115bc

1.8 ± ± ± ± ±

0.003 0.003 0.012 0.002 0.006

0.123abc 0.121abc 0.124ab 0.124ab 0.112c

± ± ± ± ±

0.008 0.009 0.003 0.006 0.004

Values are the averages ± standard deviations. Different small letters (aec) indicate significant differences between all samples (p  0.05).

3

solution, due to thermal gelation of WPI by heat treatment in the process of preparing film, resulting in strong elastic protein gel beyond this critical concentration. The viscous KGM solution had the ability to form translucent and matt film with high-strength and flexibility, even at such low concentration (1 g/100 g solution). On the other hand, WPI films were transparent, slightly yellow-tinted and glossy. To investigate the effect of WPI on properties of KGM film, WPI were incorporated into the casting solution. It was found that the maximum WPI in the casting solution of the blend film was much lower than maximum WPI concentration used to form WPI film. The maximum total amount of biopolymer in film forming solution of the blend film was 4.2 g/100 g solution, mainly limited by strong viscosity of the blended casting solution. Depending on the ratio of KGM and WPI, the resulting blend film had visual characteristics of both KGM and WPI films, varied from more translucent and matt with higher KGM, and vice versa. 3.2. Thickness The thickness of KGM, WPI and blend films are shown in Table 1. The thickness of the films varied between 0.112 mm and 0.128 mm. The thickness of WPI film was slightly lower than KGM film. For the blended film, KGM:WPI ratio and glycerol content did not significantly affect the film thickness. Thus, regardless of biopolymer type and glycerol content, controlling the total solids of casting solution per casting plate could effectively produce cast films with consistent thickness, which in turn kept the error in further property tests minimized. 3.3. Transparency Table 2 shows the calculated transparency value of the films. WPI films had the lowest transparency values, indicating that WPI films had the highest transparency, compared to blend films and KGM films. For both glycerol contents, increasing WPI content incorporated in blend films significantly decreased transparency values of blend films. This finding is consistent with previous research, in which WPI films had greater transparency than polysaccharides (methylcellulose, hydroxypropylmethylcellulose, sodium alginate) or polysaccharideseWPI blend films (Yoo & Krochta, 2011). There was no significant effect of investigated glycerol content on transparency value of the films. 3.4. Mechanical properties The effects of incorporation of different concentrations of WPI as well as different amounts of glycerol as a plasticizer on mechanical properties were determined (Fig. 1). KGM films exhibited significantly higher TS, EM and %E than WPI films indicating that KGM films were stronger and more ductile materials. Incorporating WPI into matrix of KGM films resulted in significantly decrease in TS and EM and significantly increase in %E, comparing to the pure KGM film. However, there was no significant difference in TS and EM between the blend films and WPI film. It was hypothesized to be attributed to the partial incompatibility between two biopolymers. Occurrence of the denatured whey protein aggregates might result in the discontinuity of the KGM network, thereby reducing film brittleness and improving flexibility. This finding is consistent with previous work by Yoo and Krochta (2011), which reported the decrease in tensile strength of HPMC:WPI or MC:WPI blend films with increased concentration of WPI in the blend. Significantly increased %E values were also observed as WPI concentration in the blend films decreased to 3.4 g/100 g solution,

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Transparency value Glycerol (g Gly/100 g solution) 1.5

1:0 0.8:3.4 0.6:3.6 0.4:3.8 0:4.2

0.901bc 1.206ab 0.951abc 0.882bc 0.648c

1.8 ± ± ± ± ±

0.031 0.495 0.05 0.036 0.04

0.915bc 1.264a 0.733c 0.735c 0.61c

± ± ± ± ±

0.077 0.097 0.154 0.204 0.017

Values are the averages ± standard deviations. Different small letters (aec) indicate significant differences between all samples (p  0.05).

indicating the improved stretchability compared to the pure WPI film. It was hypothesized to be due to the effect of WPI content. Generally, WPI film was more brittle than KGM film; therefore, the film with lower WPI concentration tended to have higher %E. There was a significant effect of glycerol content on %E of the blend films. The films with higher glycerol concentration (at 1.8 g glycerol/100 g solution) tended to have higher %E due to plasticizing effect. Plasticizers can reduce internal hydrogen bonding and increase the intermolecular spacing. This finding is consistent with previous work by McHugh and Krochta (1994), which found that the %E of the film increased with an increase of plasticizer concentration while tensile strength and elastic modulus decreased. 3.5. Solubility Solubility is an important property of edible films. Typically, low water solubility property is essential in enhancing film overall integrity and water resistance. However, certain applications require high solubility of the film in water such as dissolvable pouches for a pre-weighted single size serving instant powder. Solubility of film is shown in Fig. 2. The KGM film had the highest solubility value because chemical structure of KGM consists of numerous hydrophilic hydroxyl groups (Li et al., 2006). The % solubility of the WPI film was found to be the lowest indicating the strong structural cohesion of partially heat-denatured WPI matrix (Perez-Gago, Nadaud, & Krochta, 1999). The improved water resistance of the blend films was hypothesized to be due to the formation of covalent intermolecular disulfide bonds of whey proteins in the blend films. During heat denaturation, whey protein molecules unfold and expose free sulfhydryl groups which promote intermolecular disulfide bond formation, thus allowing formation of insoluble films (Floris, Velde, Bodnar, & Alting, 2010; Janjarasskul, Tananuwong, & Krochta, 2011; Perez-Gago et al., 1999). Solubility of WPI films could be manipulated by varying the availability of disulfide bonds by changing the WPI-aggregate preparation conditions; e.g. ranging from 100% solubility of native WPI to approximately 70%, 50%, 20% solubility of WPI aggregates pretreated at 68.5  C for 2 h, 90  C for 30 min and 90  C for 7 min, consecutively (Floris et al., 2010). The differences in film solubility were due to the fact that WPI was a mixture of proteins with different denaturation kinetics (Floris et al., 2010). Therefore, the incorporation of WPI decreased water solubility of KGM film, as shown by decreasing the solubility value. The solubility of WPI films (20%) is consistent with previous works (Brindle & Krochta, 2008; Perez-Gago et al., 1999; Sothornvit & Krochta, 2000). Glycerol content at 1.5 and 1.8 g/100 g solution did not apparently affect solubility of the films. Because glycerol is very hydrophilic and typically readily soluble in water, increasing glycerol in cast film is assumed to increase solubility of the films.

3.6. Water vapor permeability Water vapor permeability of KGM, WPI and blend films are shown in Fig. 3. There was no significant effect of biopolymer composition on water vapor permeability of the films. This is due to the fact that permeability coefficient (P) is a product of the diffusion coefficient (D) and solubility coefficient (S). Although incorporating WPI improved water insolubility (decreasing solubility coefficient) by providing the intermolecular disulfide bond formation, such effect was counter-balanced with plasticizing effect of WPI molecules in KGM matrix (increasing diffusion coefficient). Thus, the overall water vapor permeability was not significantly affected by incorporation of WPI.

(a) Tensile strength (MPa)

(g KGM:g WPI)/100 g solution

Laohakunjit and Noomhorm (2004) reported that solubility of glycerol plasticized rice starch film was dependent on incorporated glycerol concentration and the value of the plasticized film was significantly higher than that of non-plasticized films. Although the KGM and/or WPI films with higher glycerol concentration tended to have higher solubility value, this trend was not clearly shown for the blended films in this current study.

60 50

a

40

30 20

b c

10 0

(b) 50 45 40 35 30 25 20 15 10 5 0

c c

c

c c

0.8:3.4 0.6:3.6 0.4:3.8 0:4.2 g KGM : g WPI/ 100 g solution

a

b c c

1:0

(c)

c

c

1:0

Elastic modulus (MPa)

Table 2 Transparency value of konjac glucomannan (KGM), whey protein isolate (WPI) and blend films.

c

c

c

c

c

c

0.8:3.4 0.6:3.6 0.4:3.8

0:4.2

g KGM : g WPI/ 100 g solution 120

80

ab

de bc

60 40

a

a

100 % Elongation

4

e

cd de

20

f f

0 1:0

0.8:3.4 0.6:3.6 0.4:3.8 0:4.2 g KGM : g WPI/ 100 g solution

Fig. 1. Mechanical properties; tensile strength (a), elastic modulus (b) and (c) of konjac glucomannan (KGM), whey protein isolate (WPI) and blend 1.5 g Gly/100 g solution ( ) and 1.8 g Gly/100 g solution ( ). Error standard deviation. Bars with different letters are significantly different at

elongation films with bar shows p  0.05.

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5

90 a

80

% Solubility

70 60

b

c

50

d d

40 30

e e

20

f

e f

10 0 1:0

0.8:3.4 0.6:3.6 0.4:3.8 0:4.2 g KGM : g WPI/ 100 g solution

Fig. 2. Solubility of konjac glucomannan (KGM), whey protein isolate (WPI) and blend films with 1.5 g Gly/100 g solution ( ) and 1.8 g Gly/100 g solution ( ). Error bar shows standard deviation. Bars with different letters are significantly different at p  0.05.

Glycerol content also did not have a significant effect on WVP of the films, although the films with higher glycerol concentration tended to have higher water vapor permeability. Many research studies (Banker, 1996; Cuq, Gontard, Cuq, & Guilbert, 1997; Janjarasskul et al., 2011; Koelsch, 1994; Sothornvit & Krochta, 2000) reported the plasticizing effect of glycerol on impairing WVP of biopolymer films by reducing intermolecular forces along polymer chains and increased the polymer free volume for water molecules to migrate. However, the narrow glycerol contents investigated in this study did not significantly affect WVP of the films. 3.7. Thermal transitions

WVP (g-mm/kPa-h-m2)

Thermal transitions of KGM, WPI and blend films were investigated (Fig. 4). For the WPI and the blend films, the endotherm with To around 153e155  C, Tp around 175e179  C and DH ranged from 5.55 J/g to 10.35 J/g was detected. Hernandez-Izquierdo and Krochta (2008) also reported the existence of endothermic peaks with an onset temperature at the similar range (156.3 ± 1.4  C) for both solution-cast and extruded WPI films plasticized with glycerol. They hypothesized that this endotherm represented the transformation of the film matrix to a thermoplastic-extrudable melt. In case of KGM films, there was no endotherm in this temperature range. Degradation of WPI films was reported to occur at more than 200  C (Janjarasskul et al., 2011) while KGM film was likely to be degraded at around 326  C (Xu et al., 2007). 5 4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 1:0

0.8:3.4 0.6:3.6 0.4:3.8 g KGM : g WPI/ 100 g solution

0:4.2

Fig. 3. Water vapor permeability of konjac glucomannan (KGM), whey protein isolate (WPI) and blend films with 1.5 g Gly/100 g solution ( ) and 1.8 g Gly/100 g solution ( ). Error bar shows standard deviation.

Fig. 4. Differential scanning calorimetry thermograms of konjac glucomannan (KGM), whey protein isolate (WPI) and blend films at (a) 1.0:0 g KGM:g WPI/100 g solution, (b) 0.8:3.4 g KGM:g WPI/100 g solution, (c) 0.6:3.6 g KGM:g WPI/100 g solution, (d) 0.4:3.8 g KGM:g WPI/100 g solution and (e) 0:4.2 g KGM or WPI/100 g solution.

3.8. Seal strength Heat sealing of the films was determined near the peak temperature of the endotherm determined by DSC. Sealing temperature is considered as the most important process variable which affects seal strength. In this study, the other two heat sealing variables; jaw pressure and cooling time, were kept constant. The result showed that WPI and KGMeWPI blend film with highest WPI concentration (0.4:3.8 g KGM or WPI/100 g solution) could be optimally heat sealed at 175  C. Below this optimal temperature, the seals delaminated. Distorted seals together with film discoloration and off-odor obtained at higher temperatures could hypothesized to be attributed to degradation of whey proteins matrix corresponding to thermal transitions. During the heat sealing process, the heat melts crystalline polymer on the surfaces of two films pressed together between heated plates. The application of pressure causes the interfacial interactions to form across the joint surface. Upon cooling, a heatsealed joint, depending on the surface chemistry of the sealing material(s), is produced due to re-crystallization of the polymer (Mueller, Capaccio, Hiltner, & Baer, 1998). The main forces responsible for the sealed joint formation of WPI films reported to be hydrogen and covalent bonds; involving CeOeH and NeC (Kim & Ustunol, 2001). On the other hand, KGM films and other blend films with lower concentration of WPI were not heat-sealable. As seen from DSC thermogram (Fig. 4), those melting endotherms were mainly be contributed to the WPI portion. Therefore, less proportion of WPI might not be sufficient to create the heatsealable matrix. Li et al. (2006) also found that pure KGM film could not be heat sealed at 140  C. Although WPI films and blend film with highest WPI concentration (3.8 g WPI/100 g solution) were capable to be heat sealed at 175  C, the seal strength of such seals cannot be determined. This is

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Fig. 5. Mode of failure of heat seal during seal strength test (a) peeled seal and (b) material break.

due to the failure mode of the seal. To obtain correct seal strength, the seal must be peeled cohesively at sealing surface (shown in Fig. 5a). However, in this study the sealed test strips broke at sealing edge before the sealed area could be separated from each other by the load as shown in Fig. 5b. This means that the heat-sealed area was stronger than the film sample. Thus, the heat seal strength determination could not be achieved in this study. 4. Conclusions Incorporation of WPI significantly altered properties of KGM film. The transparency of the KGMeWPI blend films increased with an increase in the concentration of WPI. The presence of WPI molecules enhanced water insolubility of the films. However, there was no significant effect of biopolymer content on water vapor permeability of the films. WPI and blend films with the highest concentration of WPI, 0.4:3.8 (g KGM:g WPI)/100 g solution, could be heat sealed at 175  C. The range of glycerol in this study did not apparently affect properties of the films. Overall, the KGM/WPI blend films exhibited a potential application as edible food films depending on applications. Acknowledgments This research project was supported by Graduate Scholarship to Commemorate the 72nd Anniversary of His Majesty King Bhumibol Adulyadej and the 90th Anniversary of Chulalongkorn University Fund (Ratchadaphiseksomphot Endowment Fund). References ASTM. (1989). Standard test methods for water-vapor transmission of materials. Standard designation E96eE80. In Annual book of American Standard Testing Methods (pp. 730e739). Philadelphia, PA: American Society for Testing and Material. ASTM. (1997a). Standard test methods for tensile properties of thin plastic sheeting. Standard designation D882-95. In Annual book of American Standard Testing Methods (pp. 162e170). Philadelphia, PA: American Society for Testing and Material. ASTM. (1997b). Standard test methods for transparency of plastic sheeting. Standard designation D1746-96. In Annual book of American Standard Testing Methods (pp. 389e391). Philadelphia, PA: American Society for Testing and Material. ASTM. (2005). Standard test method for seal strength of flexible barrier materials. Standard designation F88-07a. In Annual book of American Standard Testing

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Please cite this article in press as: Leuangsukrerk, M., et al., Properties of konjac glucomannanewhey protein isolate blend films, LWT - Food Science and Technology (2014), http://dx.doi.org/10.1016/j.lwt.2014.05.029